Protein kinases, the largest family of human enzymes, encompass well over 500 proteins. Btk is a member of the Tec family of tyrosine kinases, and is a regulator of early B-cell development, as well as mature B-cell activation, signaling and survival.
B-cell signaling through the B-cell receptor (BCR) leads to a wide range of biological outputs, which in turn depend on the developmental stage of the B-cell. The magnitude and duration of BCR signals must be precisely regulated. Aberrant BCR-mediated signaling can cause disregulated B-cell activation and/or the formation of pathogenic auto-antibodies leading to multiple autoimmune and/or inflammatory diseases. Mutation of Btk in humans results in X-linked agammaglobulinaemia (XLA). This disease is associated with the impaired maturation of B-cells, diminished immunoglobulin production, compromised T-cell-independent immune responses and marked attenuation of the sustained calcium sign upon BCR stimulation.
Evidence for the role of Btk in allergic disorders and/or autoimmune disease and/or inflammatory disease has been established in Btk-deficient mouse models. For example, in standard murine preclinical models of systemic lupus erythematosus (SLE), Btk deficiency has been shown to result in a marked amelioration of disease progression. Moreover, Btk deficient mice are also resistant to developing collagen-induced arthritis and are less susceptible to Staphylococcus-induced arthritis.
A large body of evidence supports the role of B-cells and the humoral immune system in the pathogenesis of autoimmune and/or inflammatory diseases. Protein-based therapeutics (such as Ritaxan) developed to deplete B-cells, represent an important approach to the treatment of a number of autoimmune and/or inflammatory diseases. Because of Btk's role in B-cell activation, inhibitors of Btk can be useful as inhibitors of B-cell mediated pathogenic activity (such as autoantibody production).
Btk is also expressed in mast cells and monocytes and has been shown to be important for the function of these cells. For example, Btk deficiency in mice is associated with impaired IgE-mediated mast cell activation (marked diminution of TNF-alpha and other inflammatory cytokine release), and Btk deficiency in humans is associated with greatly reduced TNF-alpha production by activated monocytes.
Thus, inhibition of Btk activity can be useful for the treatment of allergic disorders and/or autoimmune and/or inflammatory diseases including, but not limited to: SLE, rheumatoid arthritis, multiple vasculitides, idiopathic thrombocytopenic purpura (ITP), myasthenia gravis, allergic rhinitis, multiple sclerosis (MS), transplant rejection, Type I diabetes, membranous nephritis, inflammatory bowel disease, autoimmune hemolytic anemia, autoimmune thyroiditis, cold and warm agglutinin diseases, Evan's syndrome, hemolytic uremic syndrome/thrombotic thrombocytopenic purpura (HUS/TTP), sarcoidosis, Sjögren's syndrome, peripheral neuropathies (e.g., Guillain-Barre syndrome), pemphigus vulgaris, and asthma.
In addition, Btk has been reported to play a role in controlling B-cell survival in certain B-cell cancers. For example, Btk has been shown to be important for the survival of BCR-Abl-positive B-cell acute lymphoblastic leukemia cells. Thus inhibition of Btk activity can be useful for the treatment of B-cell lymphoma and leukemia.
In view of the numerous conditions that are contemplated to benefit by treatment involving modulation of protein kinases, it is immediately apparent that new compounds capable of modulating protein kinases such as Btk and methods of using these compounds should provide substantial therapeutic benefits to a wide variety of patients.
Inhibitors of protein kinases are widely sought and small molecule compounds capable of modulating protein kinases have been reported. For example, imidazopyrazines useful as protein kinase inhibitors are reported in patent publications WO 2005/047290, WO 2005/014599, WO 2005/005429, WO 2006/099075, WO 2006/053121, and US 2006/0183746. Also reported are imidazopyrimidines and pyrrolotriazines purportedly useful as protein kinase inhibitors (see U.S. patent publication No. 2006/0084650). More recent patent publication, WO 2008/033858, discloses methods of inhibiting Btk activity with various Btk binding chemical compounds. In addition, certain imidazopyridazine and imidazotriazine compounds are disclosed in WO 2007/038314 (published Apr. 5, 2007) and WO 2008/0045536 (published Feb. 21, 2008), both of which are assigned to the present assignee. The compounds of the present invention are distinguishable from those in the aforementioned references in that they possess an aryl-substituted imidazo[1,2-f][1,2,4]triazine core.
Thus, the present invention relates to a new class of substituted fused heterocyclic compounds found to be effective inhibitors of protein kinases including Btk. These novel compounds are provided to be useful as pharmaceuticals with desirable stability, bioavailability, therapeutic index and toxicity values that are important to their drugability.